1. Field of the Invention
This invention relates to a polymeric blend which, when added to an oil will increase its viscosity, particularly at higher temperatures, and to oil compositions comprising said polymeric blend. More particularly, this invention relates to a polymeric blend which is useful as a viscosity index improver and to oil compositions comprising said polymeric blend.
2. Prior Art
As is well known, the viscosities of lubricating oils vary with temperature, and, since lubricating oils generally incur a relatively broad temperature range during use, it is important that the oil not be too viscous (thick) at low temperatures nor too fluid (thin) at higher temperatures. As is also well known, variation in the viscosity-temperature relationship of an oil is indicated by the so-called viscosity index (VI). The higher the viscosity index, the less the change in viscosity with temperature. In general, the viscosity index is a function of the oil's viscosity at a given lower temperature and at a given higher temperature. The given lower temperature and the given higher temperature have varied over the years but are fixed at any given time in an ASTM test procedure (ASTM-D2270). Currently, the lower temperature specified in the test is 40.degree. C. and the higher temperature specified in the test is 100.degree. C.
Heretofore, several methods have been proposed for improving the rheological properties of lubricating oil compositions. Generally, these methods involve the use of one or more polymeric additives. Such methods wherein the polymeric additive is a linear or branched chain polymer are taught, for example, in U.S. Pat. Nos. 3,554,911; 3,668,125; 3,772,196; 3,775,329 and 3,835,053. The polymeric additives taught in this series of U.S. patents are, generally, hydrogenated, substantially linear polymers of one or more conjugated diolefins which polymers may, optionally, also contain monomeric units of one or more monoalkenyl aromatic hydrocarbons. Polymers of the type disclosed in this series of U.S. patents are typically prepared via the anionic solution polymerization of the monomers followed by hydrogenation. A selectively hydrogenated block copolymer comprising a single styrene polymer block and a single hydrogenated isoprene polymer block, which block copolymer is within the scope of the teaching of U.S. Pat. No. 3,772,196, is available commercially and is commonly used as a VI improver.
More recently, it has been discovered that certain so-called star-shaped or radial polymers, such as those disclosed in U.S. Pat. Nos. 4,116,917 and 4,156,673 can be effectively used as VI improvers in lubricating oil compositions. The polymeric additives taught in these patents are, generally, hydrogenated star-shaped or radial polymers wherein the arms are either homopolymers or copolymers of one or more conjugated diolefins or copolymers of one or more conjugated diolefins and one or more monoalkenyl aromatic hydrocarbons or a mixture of such arms. The hydrogenated star-shaped or radial polymers may be prepared by first polymerizing the arms with an anionic initiator and then coupling the living polymeric arms with a suitable coupling agent and thereafter hydrogenating the star-shaped polymer product. A star-shaped polymer wherein all of the arms are homopolymers of isoprene, which star-shaped polymer is within the scope of the teaching of both U.S. Pat. Nos. 4,116,917 and 4,156,673, is commercially available and is commonly used as a VI improver.
Even more recently, it has been discovered that certain asymmetric radial polymers are particularly effective VI improvers. This discovery is described and claimed in copending U.S. patent application Ser. No. 071,845 filed July 10, 1987, now U.S. Pat. No. 4,849,481. These asymmetric radial polymers comprise a plurality of arms, a portion of which arms are polymers of one or more conjugated diolefins and another portion of which arms are block copolymers comprising a polymeric block containing one or more monoalkenyl aromatic hydrocarbon monomer units and a polymeric block containing one or more conjugated diolefin monomer units. These asymmetric radial polymers are prepared by coupling a suitable blend of living polymers in the same manner used, generally, to prepare star-shaped polymers and then selectively hydrogenating the conjugated diolefin portion of the radial polymer.
As is also well known in the prior art, thickening efficiency of a VI improver is an important, and frequently the principal, consideration in its selection for use as a VI improver. The VI improver's ability to maintain its thickening efficiency after it has been subjected to mechanical shear is, however, also an important consideration. In general, the thickening efficiency of any given polymeric additive will vary with polymer composition and structure, but will, generally, increase with increased molecular weight of the polymer. The ability of a polymeric VI improver, on the other hand, to maintain its thickening efficiency as evidenced by its mechanical shear stability generally improves as the molecular weight of the polymer decreases. As a result, improved mechanical shear stability (improved maintenance of thickening efficiency) is, then, generally achieved at the expense of thickening efficiency although the use of an increased amount of a polymeric VI improver will offset this loss in thickening efficiency. The use of an increased amount of polymeric additive to achieve any desired thickening efficiency is, however, accompanied with several disadvantages such as increased cost of the polymeric additive itself and an increase in certin engine deposits caused by such polymeric additives. The need, then, for an improved VI improver having improved thickening efficiency, thereby requiring a reduced amount of the improver to achieve any desired viscosity index, while maintaining an acceptable mechanical shear stability is believed to be readily apparent.